The present inventors have described previously a plethora of MEMS inkjet nozzle devices using thermal bend actuation. Thermal bend actuation generally means bend movement generated by thermal expansion of one material, having a current passing therethough, relative to another material. The resulting bending motion may be used to eject ink from a nozzle opening, optionally via movement of a paddle or vane, which creates a pressure wave inside a nozzle chamber. One such example of a thermal bend actuated inkjet nozzle device is described in U.S. Pat. No. 7,819,503, the contents of which is incorporated herein by reference.
In some circumstances, it is desirable to vary a size of ink droplets ejected from a printhead. For example, printing plain text typically requires maximum black optical density (OD) and it may be desirable to eject relatively large droplet volumes in order maximize black OD for such applications. On the other hand, photo printing typically requires high resolution printing, and it may be desirable to eject relatively small droplet volumes for such applications. Different print media types, ink types and ambient conditions may also impact on the optimum droplet volume for optimum print quality.
U.S. Pat. No. 7,997,690 describes a means of printing with variable droplet volumes by varying a hydrostatic pressure of ink supplied to the printhead. A relatively high hydrostatic pressure produces a convex meniscus in each nozzle and relatively large droplet volumes, whilst a relatively low hydrostatic pressure produces a concave meniscus in each nozzle and relatively small droplet volumes. However, varying the hydrostatic ink pressure may be problematic for several reasons: it complicates the ink delivery system and pressure regulating mechanisms; relatively high hydrostatic ink pressure may cause printhead face flooding and associated printhead maintenance problems; and all nozzles in each color plane must eject droplets of the same volume—for mixed photo and text printing, it may be desirable to eject different droplet sizes in different regions of a page.
It would be desirable to address at least some of the shortcomings described above in connection with U.S. Pat. No. 7,997,690. In particular, it would be desirable to provide an inkjet printhead comprises thermal bend actuated nozzle devices, which does not rely on variable ink pressure for varying droplet volumes.